Abstract: Quasi phase-pure (>98 wt %) MAX phase solid solution ceramics with the (ZryTi)(2)(Al-0.5,Sn-0.5)C stoichiometry and variable Zr/Ti ratios were synthesized by both reactive hot pressing and pressureless sintering of ZrH2, TiH2, Al, Sn, and C powder mixtures. The influence of the different processing parameters, such as applied pressure and sintering atmosphere, on phase purity and microstructure of the produced ceramics was investigated. The addition of Sn to the (Zr,Ti)(2)AlC system was the key to achieve phase purity. Its effect on the crystal structure of a 211-type MAX phase was assessed by calculating the distortions of the octahedral M6C and trigonal M(6)A prisms due to steric effects. The M(6)A prismatic distortion values were found to be smaller in Sn-containing double solid solutions than in the (Zr,Ti)(2)AlC MAX phases. The coefficients of thermal expansion along the < a > and < c > directions were measured by means of Rietveld refinement of high-temperature synchrotron X-ray diffraction data of (Zr1-x,Ti-x)(2)(Al-0.5,Sn-0.5)C MAX phase solid solutions with x = 0, 0.3, 0.7, and 1. The thermal expansion coefficient data of the Ti-2(Al-0.5,Sn-0.5)C solid solution were compared with those of the Ti2AlC and Ti2SnC ternary compounds. The thermal expansion anisotropy increased in the (Zr,Ti)(2)(Al-0.5,Sn-0.5)C double solid solution MAX phases as compared to the Zr-2(Al-0.5,Sn-0.5)C and Ti-2(Al-0.5,Sn-0.5)C end-members.

Abstract: Sr2CrO2Cr2As2 and Ba2CrO2Cr2As2 with Cr2+ ions in CrO2 sheets and in CrAs layers crystallize with the Sr2Mn3Sb2O2 structure (space group I4/mmm, Z = 2) and lattice parameters a = 4.00800(2) Å, c = 18.8214(1) Å (Sr2CrO2Cr2As2) and a = 4.05506(2) Å, c = 20.5637(1) Å (Ba2CrO2Cr2As2) at room temperature. Powder neutron diffraction reveals checkerboard-type antiferromagnetic ordering of the Cr2+ ions in the arsenide layers below TN1Sr, of 600(10) K (Sr2CrO2Cr2As2) and TN1Ba 465(5) K (Ba2CrO2Cr2As2) with the moments initially directed perpendicular to the layers in both compounds. Checkerboard-type antiferromagnetic ordering of the Cr2+ ions in the oxide layer below 230(5) K for Ba2CrO2Cr2As2 occurs with these moments also perpendicular to the layers, consistent with the orientation preferences of d4 moments in the two layers. In contrast, below 330(5) K in Sr2CrO2Cr2As2, the oxide layer Cr2+ moments are initially oriented in the CrO2 plane; but on further cooling, these moments rotate to become perpendicular to the CrO2 planes, while the moments in the arsenide layers rotate by 90° with the moments on the two sublattices remaining orthogonal throughout [behavior recently reported independently by Liu et al. [Liu et al. Phys. Rev. B 2018, 98, 134416]]. In Sr2CrO2Cr2As2, electron diffraction and high resolution powder X-ray diffraction data show no evidence for a structural distortion that would allow the two Cr2+ sublattices to couple, but high resolution neutron powder diffraction data suggest a small incommensurability between the magnetic structure and the crystal structure, which may account for the coupling of the two sublattices and the observed spin reorientation. The saturation values of the Cr2+ moments in the CrO2 layers (3.34(1) μB (for Sr2CrO2Cr2As2) and 3.30(1) μB (for Ba2CrO2Cr2As2)) are larger than those in the CrAs layers (2.68(1) μB for Sr2CrO2Cr2As2 and 2.298(8) μB for Ba2CrO2Cr2As2) reflecting greater covalency in the arsenide layers.

Abstract: Atomic arrangements in the mixed-valence oxide U3O7 are refined from high-resolution neutron scattering data. The crystallographic model describes a long-range structural order in a U60O140 primitive cell (space group P42/n) containing distorted cuboctahedral oxygen clusters. By combining experimental data and electronic structure calculations accounting for spin–orbit interactions, we provide robust evidence of an interplay between charge localization and the magnetic moments carried by the uranium atoms. The calculations predict U3O7 to be a semiconducting solid with a band gap of close to 0.32 eV, and a more pronounced charge-transfer insulator behavior as compared to the well-known Mott insulator UO2. Most uranium ions (56 out of 60) occur in 9-fold and 10-fold coordinated environments, surrounding the oxygen clusters, and have a tetravalent (24 out of 60) or pentavalent (32 out of 60) state. The remaining uranium ions (4 out of 60) are not contiguous to the oxygen cuboctahedra and have a very compact, 8-fold coordinated environment with two short (2 × 1.93(3) Å) “oxo-type” bonds. The higher Hirshfeld charge and the diamagnetic character point to a hexavalent state for these four uranium ions. Hence, the valence state distribution corresponds to 24/60 × U(IV) + 32/60 U(V) + 4/60 U(VI). The tetravalent and pentavalent uranium ions are predicted to carry noncollinear magnetic moments (with amplitudes of 1.6 and 0.8 μB, respectively), resulting in canted ferromagnetic order in characteristic layers within the overall fluorite-related structure.

Abstract: Two novel chromium oxide arsenide materials have been synthesized, Sr2CrO2Cr2OAs2 (i.e., Sr2Cr3As2O3) and Sr2CrO3CrAs (i.e., Sr2Cr2AsO3), both of which contain chromium ions in two distinct layers. Sr2CrO2Cr2OAs2 was targeted following electron microscopy measurements on a related phase. It crystallizes in the space group P4/mmm and accommodates distorted CrO4As2 octahedra containing Cr2+ and distorted CrO(2)As(4 )octahedra containing Cr3+. In contrast, Sr2CrO3CrAs incorporates Cr3+ in CrO5 square-pyramidal coordination in [Sr2CrO3](+) layers and Cr2+ ions in CrAs(4 )tetrahedra in [CrAs](-) layers and crystallizes in the space group P4/nmm. Powder neutron diffraction data reveal antiferromagnetic ordering in both compounds. In Sr2CrO3CrAs the Cr2+ moments in the [CrAs](-) layers exhibit long-range ordering, while the Cr3+ moments in the [Sr2CrO3](+) layers only exhibit short-range ordering. However, in Sr2CrO2Cr2OAs2, both the Cr(2+ )moments in the CrO4As2 environments and the Cr3+ moments in the CrO2As4 polyhedra are long-range-ordered below 530(10) K. Above this temperature, only the Cr3+ moments are ordered with a Neel temperature slightly in excess of 600 K. A subtle structural change is evident in Sr2CrO2Cr2OAs2 below the magnetic ordering transitions.

Abstract: Electrocatalysts play a pivotal role in advancing the application of water splitting for hydrogen production. This research unveils the potential of defective biphenylenes as high-efficiency catalysts for the hydrogen evolution reaction. Using first-principles simulations, we systematically investigated the structure, stability, and catalytic performance of defective biphenylenes. Our findings unveil that defect engineering significantly enhances the electrocatalytic activity for hydrogen evolution. Specifically, biphenylene with a double-vacancy defect exhibits an outstanding Gibbs free energy of -0.08 eV, surpassing that of Pt, accompanied by a remarkable exchange current density of -3.08 A cm(-2), also surpassing that of Pt. Furthermore, we find the preference for the Volmer-Heyrovsky mechanism in the hydrogen evolution reaction, with a low energy barrier of 0.80 eV. This research provides a promising avenue for developing novel metal-free electrocatalysts for water splitting with earth-abundant carbon elements, making a significant step toward sustainable hydrogen production.

Abstract: Elemental analysis for all of the light elements up through chlorine by elastic scattering of 16 MeV protons has been shown to be feasible. Basic problems associated with such measurements are discussed including kinematics, angular distribution, and sample backings. Spectra are presented for air particulate matter for both a total filter (Nuclepore) and a size fractionated air impactor (polystyrene backed) sample. The method is absolute, non destructive and can be used in conjunction with proton induced x-ray fluorescence to quantitatively analyze all elements in the same sample in minutes of time.

Abstract: Germanium is a widely used material for tunnel FETs because of its small band gap and compatibility with silicon. Typically, only the indirect band gap of Ge at 0.66 eV is considered. However, direct band-to-band tunneling (BTBT) in Ge should be included in tunnel FET modeling and simulations since the energy difference between the Ge conduction band edges at the L and G valleys is only 0.14 eV at room temperature. In this paper, we theoretically calculate the parameters A and B of Kane's direct and indirect BTBT models at different tunneling directions ([100], [110], and [111]) for Si, Ge and unstrained Si1-xGex. We highlight how the direct BTBT component becomes more important as the Ge mole fraction increases. The calculation of the band-to-band generation rate in the uniform electric field limit reveals that direct tunneling always dominates over indirect tunneling in Ge. The impact of the direct transition in Ge on the performance of two realistic tunnel field-effect transistor configurations is illustrated with TCAD simulations. The influence of field-induced quantum confinement is included in the analysis based on a back-of-the-envelope calculation.

Abstract: To improve the interpretation of the tunnel field-effect transistor (TFET) measurements, we theoretically identify the signatures of the ballistic band-tail (BT) tunneling (BTT) current in the transfer and output characteristics of the TFETs. In particular, we demonstrate that the temperature dependence of a BTT-dominated subthreshold swing (SS) is in agreement with the reported experimental results. We explain how the temperature dependence of the output characteristics can be used to distinguish between a current dominated by BTT and a current dominated by trap-assisted tunneling. Finally, we propose an expression that relates the energetic extension of the quasi-extended BT states in the bandgap to the onset voltage for tunneling.

Abstract: Scanning electron microscopy coupled with microanalysis (SEM-EDX) is an important analytical tool for the morphological and chemical characterization of different types of materials. In many applications, SEM-EDX elemental maps are usually used and processed as images, thus flattening and reducing the spectroscopic information contained in EDX hyperspectral data cubes. The exploitation of the full hyperspectral dataset could be indeed very useful for the study of complex matrices like soil. In order to maximize the information attainable by SEM-EDX data cubes analysis, the software package “Datamuncher Gamma” was implemented and applied to study soil aggregates. By using this approach, different phases (silicates, aluminosilicates, Ca-carbonates, Ca-phosphates, organic matter, iron oxides) inside soil aggregates were successfully identified and segmented. The advantages of this method over the common ROI imaging approach are presented. Finally, this method was used to compare different aggregates in a Cr-polluted soil and understand their possible pedological history. The present method can be used for the analysis of every type of SEM-EDX data cubes, allowing its application to different types of samples and fields of study.

Abstract: Clathrate hydrates gained significant attention as a viable option for large-scale storage of natural gas, primarily methane (CH4). Unlike employing the nanoconfinement for enhancing the nucleation sites and hydrate growth as in the porous materials, whose synthesis is often associated with high costs and poor batch reproducibility, a new approach for promoting CH4 hydrates using pure water (H2O) in an unstirred reactor packed with stainless steel beads (SSB) was proposed in this fundamental work, where the interstitial space between the beads was exploited for enhanced hydrate growth. SSB of two diameters, 5 mm and 2 mm, were used as. a packed bed to investigate their effects on CH4 hydrate formation at 273.65 K, 275.65 K, and 277.65 K with an initial pressure of 6 MPa. The thermal conductivity of SSB packing potentially aided hydrate growth by expelling the hydration heat, while, the results also revealed that driving force has a substantial impact on the rate of CH4 hydrate formation and gas uptake. The experiments conducted in both 5 mm and 2 mm SSB packed bed reactors showed a maximum gas uptake of 0.147 mol CH4/mol H2O at 273.65 K with water to hydrate conversion of 84.42% with no significant variation. The results established the promotion effect on the kinetics of CH4 hydrate formation in the unstirred reactor packed with 2 mm SSB due to the availability of more interstitial space offering multiple nucleation sites for CH4 hydrate by providing a larger specific surface area for H2O-CH4 reaction. Experiments with varying H2O content were also performed and the results showed that the water to hydrate conversion and rate of hydrate formation could be enhanced at a lower H2O content in a packed bed reactor. This study demonstrates that the use of costly or intricate porous materials can be made redundant, by exploiting the interstitial voids in packing of cheap and widely available SSB as a promising alternative material for enhancing the kinetics of artificial CH4 hydrate synthesis.

Abstract: Methane (CH4) clathrate hydrates have gained much attention in the ever-growing search for novel energy storage methods; however, they are currently limited due to their poor water-to-hydrate conversions and slow formation kinetics. To surmount these bottlenecks, significant research has been centered on the design of novel methods (porous media). In this vein, the present work explores two hydrophobic mesoporous solids, an alkyl-grafted mesoporous silica (SBA-15 C8) and a periodic mesoporous organosilica (Ring-PMO), in their ability to promote CH4 clathrates. Both materials have shown to facilitate CH4 clathrate formation at mild operating conditions (6 MPa and 269–276 K). The study revealed that the maximal CH4 storage capacities are strongly linked to the critical/optimal quantity of water in the system which was determined to be at 130% and 200% of the pore volume for SBA-15 C8 and Ring-PMO, respectively. Up to 90% and 95% of the maximum water-to-hydrate conversions were achieved in 90 min at the lowest experimental temperature and critical water content for SBA-15 C8 and Ring-PMO, respectively. At these conditions, SBA-15 C8 and Ring-PMO showed a maximum gas uptake of 98.2 and 101.2 mmol CH4/mol H2O, respectively. Both the materials exhibited no chemical or morphological changes post-clathrate formations (characterized using FT-IR, N2 sorption, XRD, and TEM), inferring their viability as clathrate promoters for multiple cycles. An integrated multistep model was considered adequate for representing the hydrate crystallization kinetics and fits well with the experimental kinetic data with a low average absolute deviation in water-to-hydrate conversions among the three distinct kinetic models analyzed. Overall, the results from this study demonstrate hydrophobic porous materials as effective promoters of CH4 clathrates, which could make clathrate-based CH4 storage and transport technology industrially viable.

Abstract: Although the analysis of thermal water by spark-source mass spectrometry (SSMS) is rather timeconsuming, it allows the detection of about 20 elements of geochemical interest down to the ppb-level. A physical preconcentration is proposed in order to collect elements having quite different chemical properties, e.g. alkalis, transition elements, and elements occurring in anionic form. The relative sensitivity factors appear to be rather independent of the salt content of the graphite electrodes. Contrary to neutron activation analysis, SSMS has a quite uniform elemental sensitivity, and allows to determine elements for which neutron activation is not suitable, e.g. Sn and Pb. The precision of SSMS is however by a factor of about 2 worse than that obtained for neutron activation.